Reliably determining the level of defocus in a printed pattern is an important aspect of photolithography. A common focus monitoring scheme employs a 90° phase shifting transition on a mask. Such a 90° phase transition may be produced by etching a region on a glass mask by a depth that produces a relative phase difference of 90° for light transmitted through the etched and un-etched mask regions. The image intensity may be reduced at the 90° phase edge relative to the clear glass regions of the mask. The location of the intensity minimum near the phase edge is known to shift from one side of its nominal position to another as the image focus is changed from negative to positive defocus. A focus monitor may track the image plane location of a printed line that straddles a 90° phase step on a mask.